1. Field of the Invention
This invention relates to trimmable and continuously tunable capacitors and more specifically to a tunable-trimmable MEMS capacitor that is insensitive to the signal voltage and mechanical noise.
2. Description of the Related Art
Process related variation exists in even the most advanced micro-fabrication facility. At most, the errors caused by process related variations can be reduced to a level that can be tolerated by the applications. As tool-manufacturing improves, fabrication facilities are becoming more advanced, and produce smaller errors. However, more advanced applications call for even tighter tolerances, and thus errors due to process related variations will always exist.
These errors exist in all components including passive elements such as capacitors. Typically, capacitors can be formed by using either a pair of parallel plates separated by a thin dielectric film or an array of such plates horizontally forming an interdigitated finger structure with a thin dielectric film between the fingers. The capacitance value is proportional to the dielectric constant of the film and the overlapping area between the plates, and is inversely proportional to the distance between the plates.
A way of post-process trimming capacitors to eliminate or reduce processing error is very useful and important.
The most commonly used procedure for post-process trimming is laser trimming to reduce the overlapping area. This procedure is rather tedious since it requires measuring each individual device while trimming. In addition, laser trimming is an irreversible procedure, and only gives the specified value at the environmental conditions under which the trimming was performed. Should the capacitance drift at a later time due to changes in temperature, humidity or some other environmental condition, it is very difficult if not impossible to reconfigure the capacitance value.
Capacitance tuning is another important function for the passive components. Unlike capacitance trimming which is more for re-configuration purposes, capacitance tuning is often used for continuous tuning of a sub-circuit such as a tunable filter, for example. The quality factor Q and the tuning ratio determine the quality and tuning range of the sub-circuit.
Currently, solid state varactor diodes are used to provide a tunable capacitor. The varactor's capacitance is set by a bias current, which is generated by a sub-circuit that can consume a significant amount of steady state power. A varactor's tuning ratio is limited, typically less than 50%, which limits its usefulness for some applications such as the frequency agile secured communications. Furthermore, the signal current applied to the varactor will affect the capacitance inducing some measure of error.
Darrin J. Young and Bernhard E. Boser, "A Micromachined Variable Capacitor for Monolithic Low-Noise VCOS," Technical Digest of the 1996 Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., pp. 86-89, 1996 discloses an aluminum micromachined variable capacitor for use as the tuning element in a voltage-controlled oscillator (VCO). This device is fabricated on top of a silicon wafer using conventional deposition techniques, and consists of a thin sheet of aluminum suspended in air above the substrate and anchored with four mechanical folded-beam suspensions acting as springs to form a parallel-plate capacitor. Because their capacitor is designed to operate at very high frequencies, in or above the 900MHz range, its capacitance is low, 2-2.5 pF, and hence has small mass. As a result, the capacitor structure is insensitive to mechanical noise due to vibration or thermal cycling, for example.
A DC control circuit applies a DC voltage V.sub.C across the capacitor that generates an electrostatic force that pulls the movable plate towards the substrate against the spring force until the opposing force reach equilibrium thereby increasing the capacitance. When the DC voltage is reduced, the spring pushes the movable plate away from the substrate until equilibrium is reestablished to reduce the capacitance. The electrostatic force F.sub.C is given by: EQU F.sub.C =(.epsilon..sub.o AV.sub.C.sup.2)/(2x.sup.2) (1)
where .epsilon..sub.o is the permittivity in a vacuum, A is the surface area of the parallel plates, and x is the plate spacing. The spring force F.sub.S is given by: EQU F.sub.S =-K.sub.X (.DELTA.x) (2)
where K.sub.X is the spring constant and .DELTA..sub.X is the deflection from the relaxed spacing. Compared to varactor diodes, this approach is amenable to monolithic integration in a standard electronic circuit process without sacrificing performance.
However, Young's parallel-plate structure has a number of drawbacks. First, the parallel-plate structure has a maximum vertical deflection of 1/3, which corresponds to a limited tuning range of at most 50%. Second, the capacitor's Q is 62 at 1 GHz, limited by the device parasitics and the amount of metal that can be deposited. Third, to isolate the DC control circuit from the signal voltage a pair of large inductors must be connected between the control circuit and the capacitor. On-chip inductors are of generally poor quality and discrete inductors reduce the overall circuit integration. Lastly, the capacitance value is sensitive to fluctuations in the signal voltage. Like the control voltage, the signal voltage generates an electrostatic force F.sub.sig =(.epsilon..sub.o AV.sub.rms.sup.2)/(2x.sup.2) where V.sub.rms is the root-mean-square value of the signal voltage, that attracts the two parallel plates towards each other, thus changing the capacitance. The surface area A, plate spacing x, and spring constant K.sub.X are the same. Thus, the deflection and, hence, capacitance error can only be controlled by the difference between the control and signal voltages. Typically, V.sub.rms is much smaller than V.sub.C so that the error is small but measurable. However, if the signal voltage a becomes large, the error can be quite significant.